The present invention relates to multiple projectile guidance. More specifically, the present invention relates to systems and methods for guiding a plurality of projectiles where GPS satellite signals are denied.
2. Description of the Related Art
Navy ships are exposed to low flying, fast, and highly maneuverable missile threats. In order to provide the ships with an effective missile defense system, high accuracy measurements of incoming missile targets and precision guidance of anti-missile projectiles are required.
Many guidance systems have been developed for projectiles. In a typical radar based guidance system, the projectile is guided to the target by guidance signals developed from tracking data obtained either by a shipboard radar system or by a radar system located totally, or partially, within the projectile. The former system is commonly referred to as a command guidance system and the latter as a homing guidance system.
In a command guidance system, a high-resolution shipboard radar system tracks both the target and the projectile, calculates the proper guidance signals for the projectile based on the generated tracking data, and transmits the signals to the projectile to enable the projectile to intercept the target.
In a homing guidance system, the target tracking radar system is located totally or partially within the projectile. An active homing guidance system uses a monostataic radar system where both the radar transmitter and receiver are located in the projectile. A semi-active guidance system uses a bistatic radar system where a radar transmitter located remotely from the projectile (such as onboard a platform, such as a ship) illuminates the target and the reflected returns are received by a receiver located on the projectile. The tracking data from the radar measurements are then used to calculate the proper guidance signals to direct the projectile to the target.
Most of the monostatic and bistatic systems are designed for use with missiles and larger caliber projectiles (greater than 3 inches in diameter), whereas the optimum caliver of projectiles for high rate-of-fire guns is generally about 1 inch in diameter. Prior art guidance systems do not work well with the 1 inch diameter projectiles. In particular, prior art approaches do not accurately measure the line-of-sight angular rate to the target and or projectile with enough precision for the application. Command guidance systems with a high resolution monostatic shipboard radar are capable of measuring line-of-sight angular rate. However, these measurements are generally not as accurate as measurements made from the projectile, as with homing guidance systems. Homing systems, however, require a radar receiver as part of the projectile. The size of the smaller caliber projectiles places a constraint on the size of the radar receiver and/or antenna on the projectile. With a small antenna, a relatively accurate range rate can be measured, but the angular rate tends to be imprecise.
One critical factor required for effective projectile guidance is an accurate measurement of the line-of-sight angular rate to the target relative to the projectiles. Guidance algorithms depend on line-of-sight angular rate information to successfully direct a projectile to its target. Poor line-of-sight angular rate measurements may cause a projectile targeting error. Such considerations are detailed in U.S. Pat. No. 6,653,972 to Krikorian and Rosen, issued Nov. 25, 2003. Hence, there is a need in the art for an improved method or system for accurately measuring line-of-sight angular rates for precision guidance of small caliber projectiles.
Projectile guidance systems need to be effective under all trajectories possible for the target as well as the projectile(s) directed to it. In some circumstances, prior art systems are vulnerable to multipath errors arising within the guidance system from the relative position of the target, the projectile(s) and the shipboard radar illuminating the target. In effect, while the targeting accuracies of the bistatic differential range rate methods offer a degree of accuracy, they rely on accurate determination of actual projectile velocities and relative positions with respect to the target and the shipboard radar. Accurate projectile velocities and positions however may be difficult to obtain in the face of various radar related limitations, such as multipath effects where the target is traveling at low altitudes.
In the parent application, the multipath effects on radar measurements are reduced by using Ground Positioning System (GPS) receivers of timing signals from geo-orbiting satellites to provide navigation information to in flight projectiles directed to a radar tracked target. In some circumstances, however, the GPS signals from the satellites may be unavailable because of interference, jamming or other causes. The lack of GPS satellite supplied signals to the projectiles may adversely affect the operation of the guidance system as their exact position may not be immediately apparent.